In wireless communication devices, radio frequency (RF) power amplifiers (PAs) are often used to provide transmit signals at increased power levels needed for operation within a communication system. For example, cellular telephone devices use PAs to transmit signals at power levels needed to communicate effectively with cellular base stations. In addition, these transmit power levels must often be controlled or limited by the communication device to meet regulatory requirements. As such, in a cellular telephone application, there is a need to detect the power output of the PA that is delivered to the antenna. In prior communication devices, a directional RF coupler has been used to split off a proportional part of the transmit output signal so that the transmit output power can be monitored by the system.
FIG. 1 (Prior Art) is a block diagram of an embodiment 100 for the use of a directional RF coupler 102 to provide information concerning delivered power back to a host processor for use in a power control loop. As depicted, a PA 104 sends a transmit signal 110 through the RF coupler 102 and through antenna switch circuitry (SW) 106 to antenna 108, which serves as the load for the transmit signal. The incident power (Pi) delivered from the PA 104 to the antenna 108 through the RF coupler 102 and switch circuitry (SW) 106 is represented by arrow 112. The RF coupler 102 includes a loop between resistors (R) 116 and 118 that provides a first output 120 that is proportional to the incident power (Pi) 112 and a second output 121 that is proportional to the reflected power (Pr) 114. Resistors (R) 116 and 118 can be coupled to ground and can be matched resistors, for example, set at 50 ohms.
Prior systems use the first output 120 from the direct RF coupler 102 to represent the power delivered to the antenna 108. This first output 120, which is proportional to the incident power (Pi) 112, is then provided to a power control loop used by host processors to control the output power to the antenna 108 for these prior systems. Because this first output 120 is at RF frequencies, an RF receiver and down-converter are used by host processors to reduce the first output signal 120 to baseband frequencies.
One problem with these prior solutions is that the actual delivered power (Pd) 122 to the antenna 108 is typically less than the incident power (Pi) 112 due to reflected power (Pr) 114. Power will be reflected back, thereby reducing the actual power delivered to the antenna 108, if there are load mismatches, such as mismatches associated with the antenna 108. Because such mismatches typically exist, the actual delivered power (Pd) 122 is the incident power (Pi) 112 reduced by the reflected power (Pr) 114 as represented by the equation Pd=Pi−Pr. As such, the output power indicator used by prior solutions is not accurate because it is proportional to the incident power (Pi) 112 and ignores the reflected power (Pr) 114. In short, the first output 120 used by prior solutions indicates a larger delivered power than is actually occurring because it is proportional to the incident power (Pi) 112 and does not consider the reflected power (Pr) 114.
Another problem with these prior solutions is that directional RF couplers are difficult to integrate because they include large passive components, high Q transformers and capacitors, or coupled transmission lines. Further, as indicated above, the output signals from directional RF couplers are at RF frequencies and require a RF receiver and down-converter in the power detection loop used by the host system to bring those signals to baseband so that they can more easily be used by the host processor to control the output power of the PA. While a prior system could use both the first output 120 and the second output 121 from the directional RF coupler 102 in an effort to determine an actual delivered power (Pd), such a solution would still be difficult to integrate due to the use of the directional coupler 102. Further, an RF receiver and down-converter would need to be provided for both the first and second outputs 120 and 121.
It is desirable, therefore, to provide a more accurate delivered power indication for systems using PAs, including such systems for cellular telephone applications. It is also desirable to have delivered power detection circuitry which can be integrated on the same integrated circuit die as the PA circuitry or other integrated circuitry or can be included within the same semiconductor package for the PA. Further, it is desirable to have delivered power detection circuitry that provides its output signals at baseband frequencies.